1. Field of the Invention
This invention is concerned with the production of high quality catalytic cracking feedstock from residual oil. In particular, it is concerned with a process in which an atmospheric tower or a vacuum tower residual oil is demetallized in a first stage treatment, and is subsequently hydroprocessed in the presence of hydrogen with a specific catalyst characterized by a particular pore size distribution to reduce its nitrogen content. The first stage treatment may be a solvent treatment such as is practiced in deasphalting, or it may be catalytic.
2. Description of the Prior Art
The art of catalytic cracking of hydrocarbons to produce gasoline and other light distillates including fuel oil from heavier fractions of petroleum has been highly developed. This art is important technologically and economically since it increases the supplies of needed light liquid fuels at the expense of heavier fractions. These heavy fractions appear to be in less demand or are replaceable by other sources of energy such as coal. The cracking process, however, has some limitations. In its best form, it requires a distillate feed which is substantially free of metals. The reason for this is that the modern highly selective catalysts used in the process are poisoned by metals introduced with the feed. These metals accumulate on the cracking catalyst and cause the production of inordinate amounts of coke and gas by-products that are generally regarded as economically undesirable. In addition, if the generation of inordinate amounts of gas overloads the gas plant associated with the catalytic cracker, the refiner may be compelled to reduce the amount of feed passed through the cracking unit. Thus, the economic penalty for excessive metals in the feed is compounded.
For the foregoing reasons, it is not common practice to put residual oils into a catalytic cracker.
Residual petroleum oil fractions produced by distillation of crude petroleum are characterized by relatively high metals, sulfur and nitrogen content. This comes about because practically all of the metals present in the original crude remain in the residual oil fraction and a disproportionate amount of sulfur and nitrogen from the original crude also remains in that fraction. Principal metal contaminants are nickel and vanadium and small amounts of iron and copper are also sometimes present. Additionally, trace amounts of zinc and sodium are found in some feedstocks.
The amount of metals present in a given hydrocarbon stream is often expressed as a chargestock "metals factor." This factor is equal to the sum of the metals concentrations, in parts per million, of iron and vanadium plus ten times the concentration of nickel and copper in parts per million, and is expressed in equation form as follows: EQU F.sub.m =Fe+V+10(Ni+Cu)
Conventionally, a chargestock having a metals factor of 2.5 or less is considered particularly suitable for catalytic cracking. Nonetheless, streams with a metals factor of 2.5 to 25, or even 2.5 to 50, may be used to blend with or as all of the feedstock to a catalytic cracker. The particular circumstances at a refinery location, including availability of chargestocks and equipment design and capacity, will dictate the maximum metals level that is tolerable, and in general this will correspond to a metals factor less than 50.
In any case, the residual fractions of typical crudes will require treatment to reduce the metals factor. As an example, a typical Kuwait crude, considered of average metals content, has a metals factor of about 75 to about 100. As almost all of the metals are combined with the residual fraction of a crude stock, it is clear that at least about 80% of the metals and preferably at least 90% needs to be removed to produce fractions with a metals factor of less than 50 suitable for cracking chargestocks.
There are several ways in which the foregoing metal and sulfur contents may be reduced in a residual oil. For example, the residuum may be destructively distilled to produce distillates of low metals content leaving behind a solid coke fraction that contains most of the metals. Coking is typically carried out in a reactor or drum operated at about 800.degree.-1100.degree. F. temperature and a pressure of 1-10 atmospheres.
Except for the residua from a limited number of paraffinic type crudes, most crudes contain considerable fractions of asphalt. The residuum may be deasphalted by employing propane, for example, as a deasphalting solvent. In this process, propane and residuum are typically introduced at different points in the side of an extraction tower. The propane, being lighter, is introduced below the oil and flows upward countercurrently to the asphalt which is rejected. Oil and propane are removed from the top of the tower and asphalt from the bottom. The two streams are transferred to recovery systems where the propane is removed and returned to the extraction system. As is known to those skilled in the art the amount of solvent used and the selection of other process variables affect the efficiency of asphalt removal. The recovered deasphalted residuum is characterized by considerably lower metal and sulfur content than the crude residuum.
In addition to the foregoing processes, it has been proposed to catalytically reduce the metals and sulfur content of residual oils. There have been a number of variations proposed for catalytic demetallation, many of which depend on selection of a catalyst having a particular pore size distribution. In general, however, the processes utilize a catalyst comprising a Group VIB and a Group VIII hydrogenation metal on an alumina support. To effect demetallation, the residual oil and hydrogen are passed at elevated temperatures of about 600.degree.-900.degree. F., at a space velocity of 0.1 to 10 LHSV (i.e., 0.1 to 10 volumes of oil per volume of catalyst per hour), and at a hydrogen pressure of about 500-3000 psig. Patents which describe such catalytic demetallation processes include: U.S. Pat. No. 4,054,508 issued Oct. 18, 1977; U.S. Pat. No. 3,730,879 issued May 1, 1973; U.S. Pat. No. 3,830,720 issued Aug. 20, 1974; and U.S. Pat. No. 3,696,027 issued Oct. 3, 1972. Applicant makes no representation that this is an exhaustive list, but it does serve to illustrate the state of the art.
The demetallized residual oil produced by any of the above processes usually still contains a substantial amount of total and basic nitrogen. These basic nitrogen compounds are deleterious in catalytic cracking because they act as catalyst poisons, reducing the activity and selectivity of the catalyst.
It is an object of this invention to provide a process for preparing high quality catalytic cracking feedstock from a residual oil. It is a further object of this invention to provide a catalytic process for treating a demetallized residual oil to reduce its nitrogen and sulfur content, thereby improving its quality as a catalytic cracking feedstock. It is a further object of this invention to provide a catalytic system for demetallizing, desulfurizing and denitrogenating a residual oil by contact with two different catalysts in series. These and other objects will become apparent on reading this entire specification including the claims hereof.